Preparation of borohydrides



United States Patent O PREPARATION OF BOROHYDRIDES Filed July 9, 1956,Ser. No. 596,415

7 Claims. (Cl. 2314) No Drawing.

This invention relates to the preparation of borohydrides by the generalreaction of hydrolysis of magnesium diboride, MgB- with bases. Moreparticularly, this invention relates to the preparation of alkali metaland ammonium borohydrides by reaction of magnesium diboride with thecorresponding bases.

It has previously been reported that the reaction of a magnesium boride,prepared from magnesium and boron trioxide, and dilute potassiumhydroxide solution produced the potassium salts of the isomericborohydrates. It has now been found, however, that the reaction ofmagnesium diboride in a relatively strong basic medium produces theborohydride of the corresponding base.

Thus according to this invention, alkali metal and ammonium borohydridesare prepared by contacting magnesium diboride with an alkali metal or anammonium hydroxide in a reaction medium such that the basicity of thereaction medium is at least equivalent to that of a 2 molar potassiumhydroxide solution in a mole ratio of hydroxide to magnesium diboride of1:1. Higher percent conversions of boron to horohydride are obtainedwith more strongly basic reaction media. In addition to water as thereaction medium, non-aqueous reaction media such as methyl and ethylalcohol are suitable. Also, some bases such as potassium hydroxide canbe reacted in the fused state with the magnesium diboride. When thereaction is carried out in an aqueous reaction medium, the reactiontemperature is preferably maintained below about 100 C.

.The borohydrides produced according to this invention are useful assources of hydrogen and as reducing agents. For example, uponacidification they readily liberate hydrogen. In dry form the alkalimetal borohydrides may be contacted with a non-aqueous acid such as BFto produce diborane. The reaction is performed in a medium of anhydrousdiethyl ether, with the usual precautions to exclude air. Diborane isuseful as a welding fuel according to the method described in US. Patent2,582,268.

This invention will be further illustrated by the following examples.The magnesium diboride employed in the examples was prepared by directcombination of boron and magnesium in a closed system at 950 C. under anatmosphere of argon. The commercial grade contains about 82.4 weightpercent magnesium diboride and the balance is essentially magnesium.

Example I 46 grams of commercial MgB (82.4%) are digested for 3-6 hoursin 21 Florence flask with 250 ml. of 8 molar KOH solution underconditions which permit the temperature of the reaction to becontrolled. The temperature is maintained at a value somewhat below 100C. When the reaction is complete, as indicated by the cessation of thecopious evolution of hydrogen, the mixture is quickly filtered. Thefiltrate which has a brownish purple color and which continues to evolvehydrogen slowly is found to contain on analysis 12.4 grams of potassiumborohydride. By fractional crystallization in 2,942,935 Patented June28, 1960 23 grams of MgB (82.4 percent) are digested for 8 to 12 hoursin 250 ml. of 3 M KOH. The reaction mixture is kept well stirred andcooled during the addition of the MgB to the base and during the firstfew hours of the reaction. Thereafter the reaction runs smoothly at roomtemperature. The water soluble fraction is rapidly filtered and slowlyevaporated under vacuum. Due to their relatively low solubility thefirst crystals are easily separated from the remainder of the solutionby filtration. Analytical data show that. this product of the hydrolysisof MgB and strong KOH was KBH (boron: calcd. 20.06 percent; found, 19.99percent). Four moles of gas per mole of KBH, are evolved uponacidification, in agreement with the equation:

The observed value of a =6.7274i0.0003 A.U. compares quite favorablywith the reported value for KBH of a =6.7274 A.U. [3. C. Abrahams and J.Kalnais, J. Chem. Phys, 22, 434 (1954)]. A total yield of 5.72 gramsKBH, is obtained, as determined by the amount of H evolved uponacidification of the solution. Other crystals which formed in thesolution are found by analysis to be a potassium borate of the formulaKBO l%H O. The powder diifraction pattern shows principal lines having dvalues of 5.5m, 3.78m, 2.97s, 2.73m, 2.48m, 2.25s, 1.85m and 1.60m.After all the KBH, is removed the filtrate is acidified with dilute HThe acidified solution still contains compounds with the ability toreduce ceric or dichromate ions, simultaneously liberating H Example III7.7 grams of MgB are reacted in ml. of 4 M (CH NOH, filtered, and thefiltrate slowly evaporated in vacuum. The first crystalline product toseparate from the solution is (CH NBH The equivalent of 4.04 gms. of (CHNBH is obtained, as determined by H yield analysis. The resultingsoluble fraction is evaporated and square platelets crystallize from thesolution. A powder ditfraction pattern of this product shows it tocrystallize in the tetragonal system. The dimensions of the primativeunit cell are This appears to be isomorphous with (CH NCl whosedimensions are:

it is quite evident therefore that the product is (CH N.BH-tetramethylammonium borohydride, with the BH, occupying the position ofthe C1 in the structure. The calculated increase in the cell dimensionswhich would result from such a substitution are in good agreement withthe observed values.

Example IV 5 grams of MgB (82.4%) and 25 grams of KOH are refluxed with270 ml. of absolute ethyl alcohol in a standard reflux apparatus for 3hours, cooled and filtered, X-ray analysis shows the presence of anamount of KBH equivalent to 15.5% conversion of the boron in theoriginal magnesium diboride into potassium borohydride.

Example V 11.5 grams commercial MgB is treated with 62.5 ml. of 4 molarNH OH and allowed to digest for 18 hours acidification.

7.98 grams of NaBH;.

at a temperature below/ 100 C. and then filtered. The

filtrate is found to contain the equivalent 0.8 gram of NH BH asdetermined by the hydrogen yield on Exa'mple VI -46 grams of commercialMgB (82.4 percent MgB are treated with 250 ml. of 4 molar NaOH and thetemperature kept below 100 C. during the reaction. After the reaction iscomplete the reaction mixture is filtered. The resulting solution isfound on analysis to contain Example VII i 23 grams of'MgB of commercialpurity (82.4 percent) are digested at a temperature below 100 C.' forthree hourswith 250 ml. of 4 molar LiOH solution. After the reaction iscomplete, the mixture is filtered and the filtrate is found on analysisto contain the equivalent of 0.093 mol or 2.03 g. of LiBH Example VIII23 grams of commercial MgB (82.4 percent) are refiuxed for 6 hours with150 ml. of a 35 percent solution of benzyl trimethyl ammonium hydroxidein methyl alcohol. The filtrate resulting from the reaction is found tocontain the equivalent of 4.38 grams of Y In general, one mole of KOHper mole of MgB is sufiicient to complete the reaction but the use ofhigher concentrations of base increases the overall yields of theborohydride. This dependency is quite reasonable, if it is assumed thatthe hydrolysis of MgB involves the initial formation of boranes, sincethe conversion of boranes Ito .borohydrides is improved by increasingthe. basic strength of the solutions. In addition, a strongly basicmedium decreases the rate of decomposition of borohydrides. Thecompeting reaction which'destroys most of hydride ion, to a morefavorable environment for the progress of desired intermediatereactions, or to other factors not at present understood. This isillustrated in part by the following table:

Percent Mole Ratio Molarity, Conver- KOH sion B to KBH 2 KOH:1 lvIgBg 811.5 2 KOH:1 MgB; 4 9.0 1 KOH:1 MgBzfl 2 8. KOH:1 MgB; 3 4. 3

It is probable that diborane (B H is one of the intermediates in theproduction of borohydrides from ing to the reaction:

4 7 magnesium diboride. Diborane reacts with water accord- (1 13 11 4110 arm-211 130 Diborane reacts with bases to form the borohydride asfollows:

(3) BO2 +4H It is evident from the above equations that the presence ofwater in the reaction medium will adversely 'atfect the yield ofborohydride. Hence, if a nonaqueous medium is used the elficiency ofconversion of MgB- to borohydride is enhanced. The examples indicatethat one could produce any borohydride from the general reaction ofhydrolysis of MgB with a suitable base. The borohydride can be isolatedfrom solution if it is stable in the basic medium at room temperatureand less soluble than its borate, also present in solution.

We claim:

1. Method for the preparation of alkali metal and ammonium borohydridesfrom magnesium diboride of the formula MgB, which comprises contactingmagnesium diboride of the formula MgB with a hydroxide selected from thegroup consisting of alkali metal and ammonium hydroxides in a mole ratioof hydroxide to magnesium diboride of at least 1:1, the hydroxide beingdissolved in a reaction medium selected from the group consisting ofwater, methanol and ethanol, and the molarity of the hydroxide solutionbeing at least 2.

2. The method of claim 1 in which the hydroxide is potassium hydroxide.

3. The method of claim 1 in which the hydroxide is lithium hydroxide.

4. The method of claim 1 in which the hydroxide is sodium hydroxide.

5. The method of claim 1 in which the hydroxide is ammonium hydroxide.

6. The method of claim 1 in which the hydroxide is V tetramethylammonium hydroxide.

7. The method of claim 1 in which the hydroxide is benzyl trimethylammonium hydroxide.

References Cited in the file of this patent Ray: Chemical Abstracts, 19;1669 (1925).

Ray: Chemistry and Industry, pp. 332-324 (1946).

Stock: Hydrides of Boron and Silicon, p. 149, Cornell University Press,1933.

Stone: Quarterly Reviews (London), vol. 9, No. 2, 1955, p. 199.

Schechter et al.: Boron Hydrides and Related Compounds, prepared underContract No. 2(S) 10992 for Dept. of Navy, Bureau of Aeronautics, byGallery Chemical Co., printed March 1951, declassified December 1953;page 53.

Markovskii: Zhur. Obshei Khim., vol. 25, pp. 433- 444 (March 1955).

Stock: .Hydrides of Boron and Silicon, 1933, p. 38.

Mikheeva et al.: Chemical Abstracts, 48; 490i (1954). I I

Jensen: A Study of Sodium Borohydride, pp. 38-48, Nyt Nordisk Forlag,Arnold Busck, Copenhagen (1954).

1. METHOD FOR THE PREPARATION OF ALKALI METAL AND AMMONIUM BOROHYDRIDESFROM MAGNESIUM DIBORIDE OF THE FORMULA MGB2 WHICH COMPRISES CONTACTINGMAGNESIUM DIBORIDE OF THE FORMULA MGB2 WITH A HYDROXIDE SELECTED FROMTHE GROUP CONSISTING OF ALKALI METAL AND AMMONIUM HYDROXIDES IN A MOLERATIO OF HYDROXIDE TO MAGNESIUM DIBORIDE OF AT LEAST 1:1, THE HYDROXIDEBEING DISSOLVED IN A REACTION MEDIUM SELECTED FROM THE GROUP CONSISTINGOF WATER, METHANOL AND ETHANOL, AND THE MOLARITY OF THE HYDROXIDESOLUTION BEING AT LEAST 2.